Self-driving Behaviors Of Liquid Droplets On Graphene Coating Gradient Textured Substrates

With the rapid development of biotechnology and micro-nanofluidic devices,the self-driving phenomenon of droplets at the nanometer scale has received more and more attentions.In this paper,the spontaneous motion behaviors of droplets on the surface of textured copper-graphene composite substrate was simulated by molecular dynamics method.A few texture models capable of driving droplets were proposed.The effect of the copper texture shape on droplet self-driving was studied,and the self-driven mechanism was explored.These research results will develop new ideas for the research and development of directional fluid transportations based on graphene-coated surfaces and nanochannels.Firstly,the effect of copper microcolumn array with area fraction gradient on the movement of nanoscale water droplets on the graphene coating was studied.The results showed that when the height of the microcolumn is high enough,the single layer graphene with periodic boundary can be flatly adsorbed on the substrate.On the composite substrate surface,the droplet can spontaneously move from lower area fraction region of the textured copper to the higher area fraction region,which mainly due to the wettability gradient of the surface of the composite substrate formed by the area fractional gradient of the textured copper.When the height of the microcolumn is short,the graphene can be adsorbed in the groove of the copper texture,thereby forming a surface with roughness gradient,and the droplet can move in the direction in which the copper texture area fraction decreases,that is,the direction in which the surface roughness of the composite substrate increases.This directional motion is attributed to the improved wettability of the hydrophilic substrate with the higher surface roughness.Secondly,the influence of the copper-graphene composite substrate with trapezoidal grooves on the movement of mercury droplets was studied.It was found that the mercury droplets could spontaneously move from the narrow end to the wide end of the trapezoidal groove.And the graphene coating can significantly improve the self-driving effect of the droplets compared with the copper grooved substrate without graphene coating.The droplet would reciprocate or break away from one side of the groove wall for one-way movement,which depending on the maximum speed obtained during the acceleration of the droplet.The velocity of the droplet would increase with increasing the groove width gradient,but the maximum displacement of the droplet would reduce.The droplet velocity and maximum displacement could both increase with increasing the groove depth.And when the droplet size increased,the droplet velocity decreased,but its maximum displacement increased.Finally,the energy method was used to calculate the total energy of the system as the droplet was adsorbed on the surface of the rectangular groove with uniform width,and the self-driving mechanism of the droplet on the surface of the trapezoidal groove was explored.The results showed that the total energy of the system composed of droplets and rectangular grooves decreased first,then increases and eventually stabilized at a certain level as the groove width increased for different groove depths and droplet sizes.That is,there is a specific groove width to minimize the total energy of the system,and the movement of the droplet on the surface of the trapezoidal groove is an oscillation process on left side and right side of the minimum total energy position.When the depth of the groove increased,the reduction of the total energy of the system increased,and the width of the groove corresponding to the minimum energy values increased,so that the maximum displacement and the maximum speed of the droplet on the surface of the trapezoidal groove increased.When the droplet size increased,the reduction of the total energy of the system and the width of the groove corresponding to the minimum energy values also both increased,so that the maximum displacement of the droplet on the surface of the trapezoidal groove increased,but the droplet velocity was not increased due to the increase of the droplet mass.